The present invention relates to the preparation of supported polycrystalline diamond or cubic boron nitride (CBN) compacts and especially to the preparation of such compacts configured for use as wire dies.
A polycrystalline compact is a sintered polycrystalline mass of abrasive diamond and/or CBN particles bonded together to form an integral, tough, coherent, high strength mass. The preparation of diamond compacts is disclosed, for example, in U.S. Pat. No. 3,141,746. CBN compacts are described, for example, in U.S. Pat. Nos. 3,136,615 and 3,233,988. A supported polycrystalline compact is a compact attached to a reinforcing or substrate material such as cemented metal carbide. In one configuration referred to as supported wire die compact, a core of polycrystalline diamond or CBN is jacketed by an annular support of, for example, cemented carbide or stainless steel.
Supported compacts often are formed in a single step in situ process such as described in U.S. Pat. Nos. 3,745,623, 3,831,428, 3,767,371, and 3,743,489. In such a process, a metal (such as cobalt) which serves as the cementing agent of the cemented carbide support flows under high temperature and pressure into the polycrystalline mass to act therein as a catalyst for the formation of crystal-to-crystal bonds. One problem which may occur in such a single step process, and especially in the production of a wire drawing die compact with an annular support, involves cementing agent/catalyst depletion at the polycrystalline diamond or CBN/support interface. Thus, in the case of a cobalt-cemented tungsten carbide supported wire die compact, if there is an excessive flow of cobalt from the carbide support ring, a depleted zone or ring may develop in the carbide accompanied by micro-cracks extending into the support material. In addition, a single step process affords no opportunity to separately inspect the unsupported polycrystalline mass. As a result, if any defect should occur in either the compact or in the support material, the entire assembly must be rejected rather than just the defective component. Similarly, system parameters in an in situ process need to be adjusted to optimize the formation of the composite whole, and cannot be adjusted to optimize the formation of the individual components. Finally, such single step processes require that both the beginning crystalline material and the support material be subjected to highly elevated temperature and pressure conditions sufficent to form the polycrystalline mass. As a result, there is a significant decrease in press throughput as compared to operations pressing just the crystalline material.
Although less frequently employed, two step processes wherein a compact is first formed and is then attached to a support also are known. Thus, the use of a brazing material for attachment purposes is described in the above noted U.S. Pat. No. 3,141,746. Similarly, supported wire die compacts made with a pre-formed cylindrical polycrystalline core around which an annular jacket of metal support material (e.g. stainless steel) is shrink-fitted in place have been used successfully. However, existing two step proceses for the formation of supported polycrystalline compacts pose certain difficulties. In the brazed approach one problem is to adequately wet the crystalline surfaces to which the support is atached. The situation is aggravated further in a more recent form of polycrystalline diamond compact termed the thermally stable compact as disclosed in U.S. Pat. Nos. 4,224,380 and 4,288,248, since a thermally stable compact can comprise a porous, nearly pure diamond material. Even in the successful shrink-fitting two-step wire die process mentioned above, it would be beneficial to improve the strength of the attachment between the compact and the surrounding metal support. In addition, shrink-fitting by its nature requires the maintenance of close tolerances between the parts to be joined. Naturally, this restriction adds to the difficulty and cost of manufacture.